A surface light source device employing a directional emission guide plate, comprising a light-scattering guide body or a light-permeable guide body and a prism sheet, has been proposed, and is widely applied for uses such as backlighting of a liquid crystal display. The prism sheet comprises a plate-like member of optical material, having a prism surface comprising a great number of prism element rows. It is known that such a prism sheet is capable of correcting propagation direction properties of light fluxes.
FIG. 1 shows a partial cutaway perspective of a diagrammatic constitution of a liquid crystal display which uses as backlighting a surface light source device of side light type, employing a conventional prism sheet. Thickness of the prism sheet 4 and other elements, as well as formation pitch, and depth and others, are exaggerated for conveniences of illustration.
As shown in FIG. 1, a directional emission guide plate 1 comprises an optical member, which is wedge-shaped in cross-section, having a light-scattering guide body or a light-permeable guide body. The scattering guide body is a known optical member which performs functions of guiding and internal scattering, comprising a matrix of, for instance, polymethyl-methacrylate (PMMA) and a "substance of different refractive index" which is mixed and dispersed uniformly in the matrix. "Different refractive index" means a substance having a refractive index which is actually different from that of the matrix.
The end face of the thick side of the guide plate 1 constitutes a light incidence surface 2, and a primary light source element (fluorescent lamp) L backed by a reflector R is provided near the incidence surface 2. A reflector 3 is provided over one major surface (under face) 6 of the guide plate 1. The reflector 3 comprises a frontal-reflecting silver-foil sheet or a diffused-reflecting white sheet. Illuminating light is extracted from the other major surface (light emission surface or light extraction surface) 5, which is the front surface of the guide plate 1. A prism sheet 4 is provided on the outer side of the front surface 5 with the prism surface facing inward.
In the explanatory cutaway portion, the outer face 4c of the prism sheet 4 is shown as an even surface. A liquid crystal panel LP is provided to the outer side of the even face 4c with a polarization separating sheet LS therebetween. The liquid crystal panel LP has a known constitution wherein a liquid crystal cell, light-permeable electrode and others sandwiched between two polarizing plates, arranged so that their axes of polarization intersect at a right angle.
The polarization separating sheet LS is an optical element in recent use, provided between the polarizing plate on the inner side of the liquid crystal panel and the prism sheet 4. This polarization separating sheet LS has high permittivity with respect to polarization components in the same direction as the polarization axis of the polarizing plate on the inner side, and high reflectivity with respect to polarization components in the direction at a right angle to the same polarizing plate.
Proposals for this type of polarization separating sheet LS include a multi-refractive polarizer (see Japan Patent Laid-Open Publication No. 4-295804), comprising a laminated arrangement of a great number of alternate layers of a first polymer substance, having a positive stress optical factor, and a second polymer substance, having a negative stress optical factor, or a polarizer made by providing alternate layers having high refractive index and low refractive index, selected to satisfy McNiall conditions, on a prism surface of one of a pair of light-permeable substrates, having isosceles-triangular prisms arranged in a straight line on one face thereof, and then optically connecting the pair of substrates with their prism surfaces facing each other (see Japan Patent Laid-Open Publication No. 6-51399), and such like.
Normally, a space (vacant layer) is provided between the liquid crystal panel LP and the polarization separating sheet LS, or between the liquid crystal panel LP and the prism sheet 4 (when a polarization separating sheet cannot be used), to prevent these elements from sticking to each other, but this space is not shown in the diagram.
The prism surface, which forms the inner surface of the prism sheet 4, has a great number of rows of prism elements. The great number of rows of prism elements are provided substantially parallel to the incidence surface 2 of the guide plate 1. As shown in the partial enlarged cross-sectional view, each row of prism elements has a pair of slopes 4a and 4b which form a V-shaped groove.
Hereinafter, in the present specification, the slope angle of the slope 4a, which faces the light incidence surface 2 of the guide plate 1, will be represented as .phi.a, and the slope angle of the slope 4b facing the opposite side will be represented as .phi.b. The slope angles .phi.a and .phi.b are defined with respect to the front surface direction (see reference numeral N). Many conventional devices use a symmetric prism sheet (i.e. .phi.a=.phi.b).
Light, sent from the light source element L into the guide plate 1, is subject to the scattering and reflection action within the guide plate 1 while being guided toward the end surface 7 of the thin side of the guide plate 1. Through this process, illuminating light is gradually emitted from the front surface 5.
As is well known, light emitted from the front surface 5 of the guide plate 1, which has received light supplied from the side, exhibits directivity of considerable clarity, and therefore such a guide plate 1 is known as a directional emission guide plate. When strong light diffusion properties are given to the front surface 5 or under face of the guide plate 1, the guide plate 1 may lose its property of directional emission. FIG. 2 and FIG. 3 are graphs illustrating emission characteristics of the front surface 5 of three example guide plates A (FIG. 2), B (FIG. 2) and C (FIG. 3).
In each of these graphs, the horizontal axis represents the direction of the brightness measurement. An angle of 0 degrees represents the frontal direction, minus represents the incidence surface 2 side and positive represents the end side (forward). The vertical axis represents relative brightness in unit (a.u.) to a peak value of 0.1. In FIG. 2, thick-line curve A represents characteristics of guide plate A, and thin-line curve B represents characteristics of guide plate B. And, in FIG. 3, curve C represents characteristics of guide plate C. The characteristics were measured using a brightness meter, to investigate brightness at different angles, in a plane perpendicular to the incidence surface 2, near a center point in the front face of guide plate. The distance from the brightness meter to the center point was 203 mm.
Each data of guide plate A to C are following:
Guide Plate A
Material: light-scattering guide body=silicon-type resin particles were uniformly dispersed at 0.08 wt % within a matrix of PMMA (polymethyl-methacrylate). The refractive index was approximately 1.5; PA1 Size: depth when viewed from the light incidence surface side was 51.5 mm, width 68.3 mm; thickness of light incidence surface side end portion was 3.0 mm, with thickness of end surface portion being 0.2 mm. PA1 Material: light-scattering guide body=silicon-type resin particles were uniformly dispersed at 0.025 wt % within a matrix of PMMA (polymethyl-methacrylate). The refractive index was approximately 1.5; PA1 Size: depth when viewed from the light incidence surface side was 190 mm, width 252 mm; thickness of light incidence surface side end portion was 3.0 mm, with thickness of end surface portion being 0.2 mm. PA1 Material: a transparent PMMA (polymethyl-methacrylate) body with a matted front face having an appropriate level of light-diffusion help increasing capability to emit light; PA1 Size: depth when viewed from the light incidence surface side was 180 mm, width 135 mm; thickness of light incidence surface side end portion was 2.5 mm, with thickness of end surface portion being 0.5 mm.
Guide Plate B
Guide Plate C
As shown in these graphs, in each case the light rays have directivity, the brightness peak (main light-beam emission angle) in each case being: approximately 63 degrees in guide plate A, approximately 77 degrees in guide plate B and approximately 72 degrees in guide plate C. Generally speaking, the emission angle which gives brightness peak increases as the depth from the light incidence surface side increases.
According to a practical criteria, the sizes of the three guide plates A, B and C correspond respectively to small, large and medium. Thus the peak angles of approximately 63.about.77 degrees, represented here, can be regarded as covering a practical range of peak angles (main light-beam emission angle). FIG. 4 is a diagram to explain the basic action of the conventional prism sheet 4 which is generally used, presupposing such directional emission of the guide plate 1.
As shown in FIG. 4, the prism sheet 4 is provided along the front surface 5 of the guide plate 1 with the prism surface facing the inner side. The top angle of each prism element forming the prism element rows is, for instance, .phi.a+.phi.b=approximately 60 degrees.
Now, when guide plate A is used, when light is supplied from the direction represented by arrow L1, the priority propagation direction of light rays emitted from the front surface 5 was therefore .theta.2=approximately 63 degrees. Considering that the refractive index of the guide plate A is around 1.5, the incidence angle to a front surface 5 results in .theta.2=being around 63.degree. and .theta.1 being=around 35 degrees. Hereinafter, in this specification, a ray corresponding to such a priority propagation direction will be referred to as "main ray". Here, the main ray is represented by reference numeral S1.
The main ray S1 emitted from the front surface 5 passes directly through the vacant layer AR (refractive index n0=approximately 1.0) and thereafter enters the slope 4a of the prism sheet 4 at an angle which is almost perpendicular thereto (.phi.a =around 30 degrees). The proportion of the main ray entering the other slope 4b is extremely low.
Next, the main ray S1 straightly led to the slope 4b by which the main ray S1 is regularly reflected. The regularly reflected main ray S1 travels in a direction which is almost perpendicular to the flat face 4c of the prism sheet 4, and is emitted from the prism sheet 4. By this process, the propagation direction of the main ray S1 is altered to a substantially frontal direction.
However, the conventional surface light source device using a prism sheet in such an aspect had the following two problems.
(1) Problem 1: The slope 4a on the main ray incidence surface side of the prism element (the slope facing the incidence surface 2 side) makes almost no contribution to producing illuminating light in the frontal direction. This signifies that there has been no technological consideration of how to use the slope 4a to contribute to producing illuminating light in the frontal direction, despite the fact that there is scope for improving the uniformity of brightness on the flat face (light emission surface) 4c.
(2) Problem 2: As shown in FIG. 1, when flat-faced elements, such as the polarization separating sheet LS, are provided in layers on the flat face 4c of the prism sheet 4, the vacant layer (generally, a layer of a different refractive index), which is provided between these elements and the flat face 4c of the prism sheet 4, is liable to become extremely thin and unstable, causing sticking between the contacting faces.
Consequently, unless the interval is maintained by using a wider space, bright and dark patterns, coloring and the likes are liable to occur as a result of interference or moares. However, maintaining such an interval is incompatible with recent strong demand for thin construction.